The present invention relates to a speed control unit for a numerically controlled machine tool of the type including two main spindles and independent spindle driving motors.
In FIG. 2, there is illustrated a conventional construction for driving the main spindles of a two-spindle numerically controlled lathe. FIG. 2(a) illustrates components for driving the first main spindle. FIG. 2(b) depicts the components which drive the second main spindle, sometimes referred to as the "auxiliary" spindle. The construction in both cases is fundamentally the same. The components on the side of the second main spindle are all marked with the subscript "a".
In FIG. 2, numerals 1 and 1a represent the main spindles: 2 and 2a are spindle driving motors; 3 and 3a are speed detecting units; 4, 4a, 5 and 5a are pulleys; 6 and 6a represent belts; 7 and 7a are chucks for holding workpieces 8, the chucks being provided at the head of each spindle; 9 is a cutting tool or the like; and 10 and 10a are spindle drive control units. The spindle drive control unit 10 comprises an adder 101 for computing a deviation between a first speed command N.sub.s1 and speed detection signals N.sub.1 transmitted from the speed detecting means 3, and a speed control circuit 102 for controlling the power supplied to the main spindle driving motor 2 by amplifying the deviation signal. The control unit 10a is likewise composed of an adder 101a and a speed control circuit 102a. In the great majority of cases, the main spindle driving motor is typically an induction motor. In this case, a known vector control method based on a slip frequency mode is adopted for control of the speed control circuits 102 and 102a, with a view to improving the respondency of the output torque.
Referring again to FIG. 2, central portions of the main spindles 1 and 1a are hollowed out to permit consecutive machining of the elongate workpieces 8 which are inserted and pass through hollows in the spindles.
In the numerically controlled machine tool, the speed commands N.sub.s1 and N.sub.s2 are issued from numerical control units which are not illustrated in the Figures.
The operation of this system in the case where the products depicted in FIG. 3(b) are consecutively produced from the elongate workpiece of FIG. 3(a) will now be described. First, as illustrated in FIG. 2, the workpiece 8 is located at the first main spindle and held by the chuck 7. The driving motor 2 is rotationally controlled, with the result that power is transferred via the pulleys 4 and 5 and the belt 6 to the main spindle 1. The main spindle is thus rotated, and the workpiece 8 is thereby rotated. While moving the tool 9 in directions X and Z, the workpiece 8 undergoes lathe turning. FIG. 4(a) illustrates the state in which the workpiece has been machined, the machined portion being indicated by vertical hatching.
Subsequently, the main spindle 1 is stopped. If necessary, a mechanism for halting the spindle at a given position is provided. An oriented stop of the main spindle may involve the use of mechanical or electrical means or both. These means are, however, not illustrated in FIG. 2, because they are not directly associated with the present invention. The second main spindle also remains rotationally stationary and fixed, in which state the second main spindle mechanism is moved in direction (-Z). The workpiece 8 is grasped by the chuck 7a when in position. Thereafter, the chuck 7 of the first main spindle is loosened, and the second main spindle la is made to retract in the direction Z. The workpiece 8 is pulled from the first main spindle 1, and then again clamped by the chuck 7 of the first main spindle 1. This state is illustrated in FIG. 4(b). The workpiece 8 is held by the chucks 7 and 7a, in which state the first and second main spindles are rotationally controlled at the same velocity. As depicted in FIG. 4(c), further machining is performed, eventually resulting in a complete product on the side of the second main spindle 1a. The second spindle 1a is then stopped, and the completely machined product is removed. Subsequently, the machining procedures described above are repeated, whereby products can successively be manufactured from the elongated workpiece.
On the occasion of performing lathe turning while simultaneously retaining the workpiece using chucks 7 and 7a of the first and second main spindles, position detecting means 11 and 11a provided at the ends of the first and second main spindles are used, which work with position comparing means 103 and 103a to, in turn, constitute a positional control loop. With this configuration, speed commands N.sub.s1 and N.sub.s2 are substituted by the outputs of units 103, 103a, and positional synchronization of the first and second main spindles is executed. The positional synchronization is effective in enhancing machining accuracy. Note that .theta..sub.s1 and .theta..sub.s2 represent the positional commands.